Fig. 1 -A nylon U.S. flag lit from the front (left)
and from behind (right).

Why is the American flag
always lit up no matter which way the sunlight comes from? Shouldn't
one side of it be shaded at least some of the time?

As shown in Fig. 1 nylon flags tend to glow when backlit. It's
sometimes difficult to distinguish direct lighting from backlighting
in photographs. This makes the flag a poor reference for lighting
direction.